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Synthesis, Fabrication, and Strain Engineering of Flexible Nanoelectronics Using Graphene and Other 2D Layered Materials

  • Author(s): Weiss, Nathan O
  • Advisor(s): Huang, Yu
  • Duan, Xiangfeng
  • et al.
Abstract

A new class of two-dimensional layered materials (2DLMs) has emerged with promising advancements made in nanoelectronics and flexible devices. Graphene, an atomically thin layer of carbon atoms in a honeycomb lattice, has gathered significant interest, followed by similar materials molybdenite, boron nitride, and black phosphorus among many others. As nanoscale building blocks, they exhibit a range of metallic, semiconducting, and insulating varieties in which to mix and match into van der Waals heterostructures (vdWHs) via controlled restacking that enables innovative nanoelectronic devices. These nanosheets behave as atomic membranes which can be stretched and bent in order to manipulate their properties using so called nanoscale origami, such as inducing a bandgap via strain engineering. A wide range of devices utilize the unique advantages of these 2DLMs for future applications such as flexible electronics, soft robotics, biocompatible interfaces and wearable technology. High-speed electronics that utilize graphene and MoS₂ are capable of outstanding performance for gigahertz regime, low-power operation using self-aligned top-gated transistors. Additionally, graphene is used as transparent, flexible, electronically tunable contact electrodes. Scalable synthesis of 2DLMs utilizes chemical vapor deposition (CVD) growth of large-area, uniform monolayer films with moderate sacrifices in electronic performance and field effect transport characteristics. Unique device designs using 2DLMs provide technological improvements in practically every subfield of electronics and beyond, yet many roadblocks remain in efforts to make use of these materials in practical and commercial applications, for instance graphene’s zero bandgap. We present two approaches towards precise application of strain in these two-dimensional membranes and observe the impact on their material properties. First, by suspending graphene across gaps and on top of topographically patterned substrates, the membrane is freestanding and capable of stretching between the peaks and valleys out of plane on the underlying surface. Using self-assembled monolayers of polystyrene spheres from 500 nm – 2 μm in diameter, O₂ plasma etching the spheres into a mask for selectively etching SiO₂ substrate into nanopillars, we create periodic structures that graphene is transferred onto. Second, elastic substrates of PDMS are prestrained, exposed to O₂ plasma, and transferred with graphene before relaxing, buckling the surface and forming ordered wrinkles and periodic, nanoscale ripples. Biaxial prestrain creates herringbone patterns, which can be precisely scaled in size from 500 nm – 3 μm wavelength with over 300 nm amplitude by controlling plasma dosage. Strained graphene structures are imaged and characterized with Raman spectroscopy mapping to analyze the peak shifting due to strain. These studies are part of a new paradigm using 2DLMs as membrane electronics manipulated through nanoscale origami for development of uniquely flexible devices.

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